In the fabrication of semiconductor devices, numerous processing steps must be performed on a semi-conducting wafer this is generally of a silicon nature. Among the numerous processing steps, a coating process wherein an insulating materials is coated on top of a wafer is frequently encountered. One of those frequently coated insulating materials is a material known as spin-on-glass (SOG).
SOG is frequently used for gap filling and planarization of inter-level-dielectrics (ILD) in multi-level metalization structures. It is a simple process and therefore is suitable for the low-cost fabrication of IC circuits. SOG materials that are commonly used can be categorized into two basic types, i.e., an inorganic type of silicate-based SOG and an organic type of siloxane-based SOG. A commonly used organic type SOG material is a silicon oxide based polysiloxane which is characterized with radical groups that replace or attach to the oxygen atoms. The basic structures, the molecular weight and the viscosity of SOG can be modified to produce SOG films of desirable properties for meeting specific requirements of certain IC fabrication process.
Typically, SOG can be applied to a pre-processed oxide surface as a liquid to fill gaps and steps on the substrate. Similar to the processing method for a liquid photoresist material, a SOG liquid can be dispensed onto a wafer and then spun-out at a predetermined rotational speed to achieve a specific thickness of the SOG layer. After the SOG liquid is evenly spun-out and applied to the surface of the substrate, it is cured at a temperature of approximately 400.degree. C. and then etched back to obtain a smooth surface. A capping oxide layer can then be deposited onto the smooth SOG surface to prepare for the deposition of next inter-level metal layer. The etch-back process is carried out so that SOG is left between metal lines as insulators but not on top of the metal. The capping oxide layer is used to fill and protect SOG insulator during further fabrication processes. For siloxane-based SOG, it has been found that gaps as narrow as 0.15 .mu.m can be satisfactorily filled. The siloxane-based SOG material therefore is suitable for 0.25 .mu.m processing technology.
The silicate-based SOG material has properties similar to that of silicon dioxide when it is fully cured. The silicate-based SOG does not absorb water in any significant quantity and is thermally stable. One significant disadvantage of the silicate-based SOG material is its large volume shrinkage after curing which leads to high residual stress and tendency to crack during curing and further processing. The cracking of a SOG layer can cause serious contamination problems for the wafer fabrication process, even though the problem can sometimes be controlled by the application of only a very thin layer, i.e., as thin as 1,000 .ANG. of the silicate-based SOG material.
In a conventional SOG coating process, a solvent edge bead rinse and a solvent backside rinse are normally incorporated into the process recipe for removing unwanted SOG deposited on the wafer edge and on the backside of the wafer. Either deposition is undesirable since they interfere with the operation of a wafer clamp which is frequently utilized in various process machines. The SOG material deposited on the wafer edge may crack under the clamp and creates a major contamination source. This is shown in FIG. 1. In a conventional SOG deposition process 10, as shown in FIG. 1, a SOG liquid material is first dispensed in step 12 by depositing a predetermined amount of liquid SOG at or near the center of the wafer. The amount of the SOG liquid can be suitably controlled by adjusting the flow rate through a dispensing nozzle from which the SOG is dispensed. The flow rate can, in turn, be controlled by a pressure existing in a liquid SOG reservoir tank.
The wafer turns on a wafer pedestal at a rotational speed between 2000 and 3000 RPM when liquid SOG material is dispensed at the center of the wafer. The liquid SOG material is therefore spun-out in step 14 by centrifugal forces from the center toward the edge of the wafer uniformly over the entire wafer surface. After all the liquid SOG material is spun-out and the edge of the wafer is fully covered, the solvent contained in liquid SOG has at least partially vaporized and formed a solid SOG coating on the wafer surface. After the spin-out step 14 is completed, an edge bead rinse process of step 16 is carried out at the edge of the wafer surface, i.e., an area of approximately 2.about.3 mm from the edge of the wafer, to wash away SOG deposited at such area. At this stage of the process, the SOG material has mostly solidified and thus the edge bead rinse process is not always effective. For instance, as shown in FIGS. 5A and 6A, the edge bead rinse process create a hump of SOG material (as indicated by the peaks in FIGS. 5A and 6A), since under an impinging jet of cleaning solvent, some of the SOG material is pushed back toward the center of the wafer even though the remaining SOG material is washed away. The amount of SOG that is pushed back toward the center of the wafer forms a hump or a new edge bead as indicated by peaks shown in FIGS. 5A and 6A. Residual processing stress left in the SOG layer after curing can cause cracks of the edge hump. The mechanical stress imposed on the edge bead by the application of a wafer clamp in a later fabrication step may also crack the hump and thus causing serious particulate contamination problems. After the edge bead rinse step 16, the backside of the wafer is rinsed by a different jet of cleaning solvent to wash away any SOG deposited at undesirable locations. This is shown as step 18 in FIG. 1. The wafer is then spun-dry in step 20 to complete the SOG coating process.
The conventional SOG coating process, shown in FIG. 1, presents serious problems in a wafer fabrication process due to the high likelihood for particulate contamination. The edge bead rinse process 16, while effective in eliminating SOG materials located in a narrow band along the edge of the wafer, creates problem of a new hump formation at the edge of the SOG layer on top of the silicon wafer.
It is therefore an object of the present invention to provide a method for depositing a coating layer on a wafer without edge hump formation that does not have the drawbacks or shortcomings of the conventional edge bead rinse process.
It is another object of the present invention to provide a method for depositing a coating layer on a wafer without edge hump formation that does not require major modifications in the process recipe.
It is a further object of the present invention to provide a method for depositing a coating layer on a wafer without edge hump formation by integrating an edge bead rinse step with a coating spin-out step.
It is another further object of the present invention to provide a method for depositing a spin-on-glass layer on a wafer without edge bead formation by carrying out a coating spin-out step simultaneously with an edge bead rinse step.
It is still another object of the present invention to provide a method for depositing a coating layer on a wafer without edge hump formation by rinsing away an edge portion of the coating material while the material is still in a liquid state during a coating spin-out process.
It is yet another object of the present invention to provide a method for cleaning a wafer edge and preventing edge bead formation by injecting a cleaning solvent at an edge portion of the wafer simultaneously during a coating spin-out process.
It is yet another further object of the present invention to provide a method for cleaning a wafer edge and preventing an edge hump formation during a wafer coating process by simultaneously washing the wafer edge with a cleaning solvent and spinning the wafer to spread out the coating material.